BEIRUT —Fighting erupted in the southern Lebanese town of Sidon on Tuesday between the supporters of a Salafist sheik and local Shiites affiliated with the Hezbollah movement in the latest example of sectarian tensions fueled by the conflict in neighboring Syria.

Gunmen using small arms and rocket-propelled grenades traded fire for two hours in a neighborhood around the mosque of Sheik Ahmad al-Assir, who has emerged as a leading voice of Sunni radicalism in Lebanon. The state-run National News Agency said one person was killed and four were injured.

The clash was typical of the violent incidents that have flared elsewhere in the country in recent months under the polarizing influence of the Syrian war. But it was the worst fighting in nearly a year in Sidon, a sleepy seaside town 25 miles south of Beirut, suggesting that the friction is spreading.

Tensions also have soared in eastern Lebanon’s Bekaa Valley, where adjoining Sunni and Shiite communities have become entangled in the war across the border. Rockets thought to have been fired by rebels in Syria have exploded in the mostly Shiite town of Baalbek, and Syrian helicopter gunships have struck the Sunni border town of Aarsal, which has become a hub for rebel Syrian fighters and refugees.

The Lebanese army sent reinforcements to the area this week after four Shiites were fatally shot while driving near the border, the most serious incident yet in a string of tit-for-tat kidnappings and assassinations that threaten to escalate into war.

The Sidon clash flared after a vehicle belonging to Assir’s brother was vandalized, according to local news reports. Assir kept his followers updated on the fighting with Facebook and Twitter postings.

“Missiles are being fired at our mosque from . . . Harat Saida, whose residents are mostly Shiite supporters of Hezbollah. Pray for us,” he tweeted.

In a recent interview, Assir said he was planning to form an armed militia, the Free Resistance Battalions, “to defend ourselves and not to attack.”

Under the peace accord that ended Lebanon’s 15-year civil war in 1990, Hezbollah is the only militia legally allowed to carry arms.